The present invention is directed to a method and a system for generating a position error signal for controlling positioning of a data transfer head in a magnetic medium recording system. More specifically, the present invention relates to a method for use in current servo systems. More specifically, the present invention relates to a servo recording and reading method which is robust against signal dropouts.
Servo tracks have been in use for many years for data recording on magnetic disk files and magnetic tape. Of course, various other implementations exist.
For magnetic disks, a short overview can be found in G. Tyson Tuttle: "Disc Drive Servo Techniques," (Mead Microelectronics Inc. course, `Electronics for Magnetic Storage Devices,` Austin, Tex., Oct. 9-13, 1995). Disks most often have embedded servo in servo sectors interleaved with data sectors. These servos are, therefore, sampled data systems. A typical sampled servo field format for a set of four data tracks consists of a gap to the data sector, a preamble used for AGC, sync mark, Gray code, constant-frequency bursts A, B, C, D recorded in a pattern and defining the actual radial servo information and a gap to the data. See Tuttle for a further explanation regarding a sampled servo field format.
Of particular relevance to the present invention is the sampled data system, the encoding in the Gray code field of various servo-related information in the form of sector address, track identification number, head number etc., and the method of extracting the read head position error signal (PES) from the amplitudes measured of the constant-frequency bursts A, B, C and D occurring one after each other. Especially, the exact sampling point in time of these frequency bursts is determined by the sync mark.
The servo information needed for a data track or a set of data tracks in the case of multichannel recording on tape is, therefore, usually a carrier recording of a pattern which defines at least two servo track center lines along the longitudinal length of the tape. For disk file servos, circles apply. The servo information content is, therefore, very low. The servo lines or circles are thus fixed references for a closed loop regulator, i.e. a servo.
For data storage on magnetic tape, other types of servo systems are in use or have been proposed. These can be divided into servo methods for helical scan tape drives and for longitudinally recorded data tracks, for example, servo tracks used for serpentine recording of one or more data channels in tape drives.
A common principle to most of these servo systems which use signal amplitudes to derive the servo PES is that, if the signal either is continuous in time or has a single measurement sample per servo PES sample, two or more frequencies are used to determine the polarity of the PES. On the other hand, if two or more samples are used to calculate a single PES sample, one single wavelength can be used, and the polarity of the PES is determined from the servo pattern. In this regard, see Tuttle. Another example is the servo recording used in the 5-1/4" data cartridge for 1/4" tape. In this regard, see U.S. Pat. Nos. 5,394,277 and 5,568,327, both to Pahr et al. FIG. 2 of each patent teaches a single wavelength that is used, and the PES polarity is determined when the correct track number is known.
Other servo systems use pulse-time information to derive the PES. These methods will not be further described here, as they are less relevant to the present invention.
When two or more wavelengths are used to record servo information and when the servo signal is extracted from the amplitudes of the servo carriers sampled at the same time, an inherent problem exists due to the well-known wavelength-dependent spacing loss, or "Wallace loss". In this regard, see Finn Jorgensen: The Complete Handbook of Magnetic Recording (3.sup.rd ed., TAB Professional and Reference Books, Blue Ridge Summit, Pa.), page 119. Spacing losses are 55-d/.lambda. dB, where "d" is the spacing and ".lambda." is the wavelength. For example, if two wavelengths are used for the servo carriers so that .lambda..sub.2 =2.multidot..lambda..sub.1 and the normal spacing is d.sub.1 =.lambda..sub.1 /5 without any signal drop out, the spacing losses are normally 11 dB for .lambda..sub.1 and 5.5 dB for .lambda..sub.2. The difference of 5.5 dB in the wavelength response must be equalized or accounted for during a servo sample. However, during a signal dropout, the spacing could be d.sub.2 =2.multidot.d.sub.1. Spacing losses are then 22 dB for .lambda..sub.1 and 11 dB for .lambda..sub.2.
Assuming that a fixed equalization of 5.5 dB has been used, the measurement error is 5.5 dB when the servo sample occurs during, for example, signal dropout. This is a significant error, and it is inherent to the magnetic reading process and to the use of two or more wavelengths for the servo. Note that the actual spacing distance "d" is a random variable for signal dropouts, and it cannot be predicted for each servo sample. However, if the sampled amplitude of a third wavelength .lambda..sub.3, which is not sensitive to the transversal or radial position of the servo read head, has a simultaneous measured change from its normal average value, the change in spacing can be calculated for this wavelength by a signal processor. When the actual change in spacing is known, the zero-PES values for .lambda..sub.1 and .lambda..sub.2 can be calculated. This is a difficult and computational intensive task to implement, and it causes limitations in the sample rate of the servo. Also, since amplitudes are measured by averaging over several wave periods, the method is not accurate for a non-linear spacing loss.
If two or more wavelengths are used to record the servo information, a calibration problem is present due to the wavelength-dependent response of the read head. Two different wavelengths generally have different amplitude responses. It is seldom that these responses are perfectly equalized so that the deviation can be ignored. Each read head must be precisely calibrated for a wavelength response if two or more wavelengths are used for the servo information carrier. Furthermore, due to read head wear, recalibration may be needed. In fact, the short wavelength response of a read head improves after a certain wear time. At the end of a life time for the read head, the short wavelength response finally degrades.
Choosing a single wavelength to write a servo pattern as commonly used for a hard disk and for the 5-1/4" data cartridge for 1/4" tape, spacing losses change over the pattern during a signal dropout and cause significant errors when the PES is calculated. The reason for this is that the pattern extends over a longitudinal distance or over a sector. Hence, a need exists for a method which extracts the servo information from a single position on the magnetic media.
Using magneto-resistive (MR) read heads, the servo information should not be degraded much during MR noise or so-called "thermal hits" when asperities in the magnetic media cause local heating of the tiny MR elements and a shift in the element's resistance.
Another need exists to increase the sample rate of the servo information. Ideally, to be more robust against built-in random defects in the magnetic media and also long-term wear of the media, the servo information should be recorded in a continuous manner along the longitudinal or circular servo tracks and also distributed transversely or radially over more than one servo track. This is not the case for a sector servo on current disk files. The accuracy of a sampled servo system often suffers because the sampling rate often ends up as a compromise between sampling rate and time needed for computing and validation of the servo samples.
Precision servo systems require recording of the servo tracks during manufacturing of the tape or disk. The servo information recorded should be of a fixed type not to be changed later during the use of the tape, or redundant servo information should be provided so that the disk or tape is robust against damage in servo areas.
Quick file access is also an important need so longitudinal position information (LPI) should be recorded as part of the physical servo recording. For disk files, such information is included in the Gray code field. For magnetic tape, if such longitudinal information is recorded, it also opens up possibilities for implementing longitudinal tape position servo control also during data file acquisition or for data file writing. In fact, the tacho signal generator in the capstan motor can be removed, and the true speed of the tape can be determined from the LPI.
Identification of the track number (track ID) or track set number is needed for feedback of the correct read head position after servo lock to a desired track or track set. Similar to the Gray code pre-written in the servo sectors in the disk files, magnetic tape used for serpentine recording must include track ID. The track ID should be recorded on the same write head pass as used for recording the servo signals. The reason for this is that, if the servo information is first recorded and then on a second pass the track ID is recorded, there is a certain probability that the track ID is written on the wrong track, and there is a need to verify that the track ID actually was written on the correct track.
Hereinafter, Track ID, LPI and other signals, for example, Manufacturer's ID (MID) and signals defining the Center of Tape (COT) area, are called servo data signals.